1. Field of the Invention
The present invention relates to a chromatic dispersion compensation controlling system in an optical communication system.
2. Description of the Related Art
Along with the recent rapid increase of the amount of communication, a larger network capacity has been demanded. Currently, a wavelength-division multiplexing (WDM) transmission system with a transmission capacity per channel of 10 Gb/s (gigabit/second) has been put into practical use. However, a far larger capacity will be needed in the future, and if the efficiency in use of frequency, equipment cost, etc., are taken into consideration, the realization of a super-high speed optical transmission system with a transmission capacity per channel of 40 Gb/s or more is expected.
However, since in such a super-high speed optical transmission system, the influence on transmission quality of waveform degradation due to chromatic dispersion, polarization mode dispersion, etc., increases, the transmission distance of optical signal is restricted, which is a problem. Therefore, in order to realize a super-high-speed optical system, an automatic dispersion compensating system for detecting a change in chromatic dispersion and polarization mode dispersion and compensating for them with high accuracy is needed.
Generally, the factor which causes the degradation of transmission quality in an optical communication system, for example, as shown in FIG. 1, is largely classified into two categories; the degradation of an optical signal-to-noise ratio (OSNR) due to the attenuation of signal power or the increase of noise power, and the degradation of the shape itself of a waveform. Furthermore, as the factors of the latter waveform degradation, there are-chromatic dispersion, polarization mode dispersion (PMD), a non-linear effect and the like.
Here, chromatic dispersion is described in detail. In an optical communication system with a transfer rate of 10 Gb/s or over, tolerance to chromatic dispersion is remarkably small. For example, chromatic dispersion tolerance in a system in which 40 Gb/s NZR optical signal is transmitted, is 100 ps/nm or less. Generally, the distance of the repeater span of an optical communication system is not constant. Therefore, for example, in the case of a system using 1.3 μm zero-dispersion single mode fiber (SMF) with a chromatic dispersion value of 17 ps/nm/km, the above mentioned chromatic dispersion tolerance deviates even if there is a difference of several km in the length of repeater span.
However, since the distance of each repeater span and chromatic dispersion value of an optical fiber transmission line possessed by a communication carrier are not accurately known, in many cases, it is difficult to realize chromatic dispersion compensation with high accuracy by a fixed dispersion compensating method using a dispersion compensating fiber (DCF). Furthermore, since a chromatic dispersion value varies under the influences of the temperature, stress, etc., of an optical fiber as time elapses, the chromatic dispersion value must be optimally adjusted not only at the time of the operation commencement but also during the operation of the system by strictly measuring chromatic dispersion.
For example, the chromatic dispersion value of a 500 km transmission line using a DCF as an optical fiber, that is caused when temperature change is 100° C. is as follows:(Chromatic dispersion)=(temperature dependence of zero-dispersion wavelength)×(temperature change of transmission line)×(dispersion slope of transmission line)×(transmission distance)=0.03 nm/° C.×100° C.×0.07 ps/nm2/km×500 km=105 ps/nm
The above-mentioned chromatic dispersion value is almost the same as the chromatic dispersion tolerance obtained when 40 Gb/s NZR optical signal is transmitted. Therefore, the automatic chromatic dispersion compensating system controlling the amount of chromatic dispersion compensation by always monitoring the chromatic dispersion characteristic of a transmission line is indispensable not only in a system using an SMF for a transmission line but also in a system using a 1.55 μm zero-dispersion shifted fiber DSF or a non-zero dispersion shifted fiber (NZ-DSF) for a transmission line.
Next, polarization mode dispersion is described in detail. Polarization mode dispersion is caused by a difference in a propagation delay time between the polarization components of an optical signal (for example, two axes, one is called slow axis and the other is called fast axis), which can be caused in all optical fibers. Generally, the higher the transfer rate of an optical signal is and the longer the transmission distance is, the greater is the influence of this polarization mode dispersion, which cannot be neglected.
There is an optical fiber with a polarization mode dispersion value of over 1 ps/km1/2 (pico-second/kilometer1/2) per unit length in optical fibers constituting old optical transmission lines laid outside Japan. If, for example, 50 km short-distance transmission is conducted using such an optical fiber, light delay difference Δτ that occurs between the two polarization components of transmitted light is 7 ps or more although one corresponding to one time slot of 40 Gb/s NRZ optical signal is 25 ps. Therefore, as in the case of the chromatic dispersion described earlier, transmission distance is restricted by the influence of polarization mode dispersion. Actually, since materials that cause polarization mode dispersion, such as an optical amplifier, chromatic dispersion compensator, etc., must be used in the transmission line of an optical communication system, there is a possibility that the transmission line of optical signal may be further restricted. Besides, since polarization mode dispersion indicates a change with the lapse of time due to a change in stress or temperature applied to an optical fiber, the polarization mode dispersion of the transmission line must be monitored not only at the time of installation but also during operation, and must be dynamically compensated.
As described above, both chromatic dispersion and polarization mode dispersion are major factors that restrict the performance of an optical communication system, and in order to improve the performance of the optical communication system, a system for dynamically compensating for these segments of dispersion, that is, an automatic dispersion compensating system, is needed. Three key technologies for realizing the automatic dispersion compensation are summarized as follows:    (a) Realization of a variable dispersion compensator    (b) Realization of monitoring the dispersion value of a transmission line    (c) Realization of a feedback control method for optimizing the amount of compensation by a variable dispersion compensator
As to the variable dispersion compensator mentioned in (a) above, there are a virtually imaged phased array (VIPA) disclosed in non-patent document 1, a tunable ring resonator disclosed in non-patent document 2, and a fiber bragg grating (FBG) disclosed in non-patent document 3, all of which are listed up as a prior art reference later.
As to the polarization mode dispersion compensator, a polarization mode dispersion compensator provided with a polarization controller (PC) at the transmitting end of an optical signal, feeding back a transmission characteristic from the receiving end and controlling so as to make the split ratio γ of light intensity into to two polarization modes 0 or 1 is disclosed in non-patent document 4, is so far proposed. A polarization mode dispersion compensator provided with a polarization controller and a polarization maintaining fiber (PMF) at the receiving end of an optical signal, giving the delay difference with an opposite sign to an optical transmission line between two polarization modes is also disclosed in non-patent document 5. Furthermore, a polarization mode dispersion compensator provided with a polarization controller, a polarization beam splitter (PBS), photo-diodes each receiving one of the two optical signal components split by this polarization bean splitter and a variable delay element that gives a delay difference between two electric signals obtained by these photo-diodes, controlling the polarization controller and the variable delay element is in non-patent document 6.
As to monitoring a dispersion value mentioned in (b) above, as a conventional measuring method of a chromatic dispersion value, a pulse method and a phase method for inputting a plurality of segments of light each with a different wavelength and measuring a group delay between a plurality of segments of output light and a phase difference, respectively, are proposed. However, in order to always measure chromatic dispersion without degrading communication quality during system operation using these measuring methods, one set of chromatic dispersion measurement equipment must be provided for each repeater span, and also measurement channel with a wavelength different from that of a data signal must be wavelength-division multiplexed, which are problems. Therefore, such measuring methods are not practical from the viewpoint of economy and equipment size.
As to a chromatic dispersion monitoring method, a method for monitoring chromatic dispersion using the intensity of a specific frequency component in a received base-band signal, utilizing its property that the intensity of a specific frequency component varies depending on waveform distortion is disclosed in non-patent document 7 in order to solve the problems. A method for monitoring chromatic dispersion, based on a bit error rate that is detected by an optical receiver, etc., is also disclosed in parent documents 1 and 2. Furthermore, there is a method for providing comparison DEC in addition to a DEC (DFF) performing the decision process of a main signal and detecting a change in chromatic dispersion is disclosed in non-patent document 8.
As to a monitoring method of polarization mode dispersion, for example, an extinction method (Senarmone method), a rotational analyzer method, a rotational phase-shifter method and a phase modulation method are known. As the representation (expression) method of a polarized state, methods using Poincaré sphere, Jones' vector, Stokes' vector, etc., are proposed (for example, see non-patent document 9). Specifically, a measuring method of polarization mode dispersion using Jones' vector and its device are disclosed in patent document 3.
Prior art technical documents related to the present invention are introduced below.
Patent document 1: Japanese Patent laid-open No. 2001-77756
Patent document 2: Japanese Patent laid-open No. 9-326755
Patent document 3: Japanese Patent laid-open No. 9-72827
Non-patent document 1: M. Shirasaki et al., “Variable Dispersion Compensator using the Virtually Imaged Phased Array (VIPA) for 40 Gbit/s WDM transmission System”, ECOC2000, PD Topic 2, 2.3
Non-patent document 2: F. Horst et al., “Tunable Ring Resonator Dispersion Compensator realized in High Refractive-index Contrast Technology”, ECOC2000, PD Topic2, 2.2
Non-patent document 3: J. A. J. Fells et al., “Twin Fiber Grating Adjustable Dispersion Compensator for 40 Gbit/s”, ECOC2000, PD Topic2, 2.4
Non-patent document 4: T. Ono et al., “10 Gb/s PMD Compensation Field Experiment over 452 km using Principal State transmission Method”, OFC2000, PD44
Non-patent document 5: Takahashi et al., “Automatic Compensation Technique for Timewise Fluctuating Polarization mode Dispersion in In-line AmplifierSystems”, Electro. Lett., Vol.30, No.4 (1994), pp. 348-349
Non-patent document 6: Takahashi et al., “Polarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systems”, J of Lightwave Tecnol., Vol.12, No. 5(1994), pp. 891-898
Non-patent document 7: Y. Akiyama et al., “Automatic Dispersion Equalization in 40 Gbit/s Transmission by Seamless-switching between Multiple Signal Wavelengths”, ECOC'99, pp. I-150-151
Non-patent document 8: S. Kuwabara et al., “Study on Dispersion Fluctuation Monitoring Method applied to Adapted Dispersion Equalization”, Journal of Comprehensive Meeting of The Institute of Electronic Information and Communications Engineers, B-10-152 (1997)
Non-patent document 9: “Representation Method and Measuring Method of Polarized State”, OPTRONICS (1997), No.5, pp.109-117
However, the above-mentioned conventional dispersion monitoring technologies have the following problems. Specifically, as shown in FIG. 2, a method for detecting the intensity of a specific frequency component in a received base-band signal and monitoring chromatic dispersion, disclosed in Non-patent document 7, etc., so called a clock monitoring method, includes a wide-band photo-diode (PD) and a clock amplifier corresponding to a 40 GHz frequency band, etc., and needs a configuration close to a main signal system optical receiver unit (O/E). Since particularly in order to separately compensate for dispersion for each channel in a WDM system, a clock monitor corresponding to each channel is needed, cost and size increase, which is a problem. In order to solve this problem, the integration of a main signal system and a monitoring system is effective. However, since it is very difficult to realize a high-accuracy band-pass filter (BSP) extracting a clock component from a received base-band signal in an integrated circuit, it is not practical, which is also a problem.
Since in the case of a SONET/SDH signal, dispersion monitoring can be realized by using a supervisory byte included in a section overhead, such as a byte B1, etc., without adding a new configuration to an existing system, and since in the case of a system adopting forward error correction, dispersion can be realized by using the correction information of FEC-IC, without adding a new configuration to an existing system, a method using a parameter indicating transmission quality, such as the error rate, Q factor, etc., disclosed in Patent documents 1 and 2 for the monitor of chromatic dispersion, has a great advantage in cost and size. However, as shown in FIG. 1, since the parameter, such as an error rate, etc., is affected by both waveform degradation and OSNR degradation, it is difficult to detect a change in a factor that causes waveform degradation, such as chromatic dispersion with high accuracy, which is also a problem.
Furthermore, since the method for detecting a change in chromatic dispersion using the DEC (DFF) disclosed in Non-parent document 8, etc., needs a high-speed circuit as fast as a main signal system circuit as a monitor system circuit, it is difficult to realize such a circuit and to ensure a desired monitor characteristic, which is also a problem. Specifically, as shown in FIG. 3, in a system adopting the above-mentioned monitoring method, comparison DECs 101 and 102 detecting a change in chromatic dispersion are added in addition to a main signal DEC100, and a decision voltage Vm (=Vth+ΔV) shifted from an optimal decision threshold Vth to a mark signal side and a decision voltage Vs (=Vth−ΔV) shifted to a space mark side are set in the comparison DECs 101 and 102, respectively. In the detection of a change in chromatic dispersion during system operation, firstly, a received main optical signal is amplified after optical/electric conversion and is distributed to each DEC in which each decision voltage is set. Then, the exclusive OR of the respective data outputs of the main signal DEC100 and comparison DEC101 is computed by an EXOR 103, and the exclusive OR of the respective data outputs of the main signal DEC100 and comparison DEC102 is computed by an EXOR 104. The EXORs 103 and 104 generate pulse outputs if two inputted data do not coincide. The respective pulse outputs from the EXORs 103 and 104 are counted in counters 105 and 106, respectively, and by monitoring the counted numbers by a controller 107, the direction of change of chromatic dispersion is detected. Therefore, all the circuits of the comparison DECs 101 and 102, EXORs 103 and 104, and counters 105 and 106 need a high-speed circuit as fast as the main signal system circuit, thus causing the problems described earlier.
Typical communication speed per wave in a current wavelength division multiplexing system is 10 Gb/s. However, aiming at the further increase of communication capacity, an attempt is being made to realize a communication speed per wave of 40 Gb/s. In the course of the development of a wavelength-division multiplexing optical communication system with communication speed per wave of 40 Gb/s, it is found that in order to realize the wavelength-division multiplexing optical communication system with communication speed per wave of 40 Gb/s, the management of chromatic dispersion must be controlled more strictly than ever.
A method for performing the feedback control of a variable dispersion compensator using transmission quality information, such as an error rate, a Q factor, etc., has been so far proposed. Particularly, a method using information similar to an error rate is considered to be advantageous in cost.
FIG. 4 shows a configuration using an error rate as a chromatic dispersion monitor as one example of conventional configurations. FIG. 5 shows its operation.
In the conventional chromatic dispersion compensation controlling system shown in FIG. 4, an optical signal transmitted from a transmitter 100 is transmitted through a transmission line 110, is amplified by an optical amplifier 120 and is inputted to a variable dispersion compensator 130. The chromatic dispersion of the optical signal is compensated by a variable dispersion compensator 130. Then, the optical signal is converted into an electric signal by an optical/electric converter 140, and is received by a receiver 150. In the receiver 150, the error rate or similar information of the received signal is computed, and is transmitted to the control circuit 160 of the variable dispersion compensator 130. The control circuit 160 controls the amount of dispersion compensation of the variable dispersion compensator 130, etc., based on this error rate or similar information. In this case, the error information detecting function of the receiver 150 acts as a chromatic dispersion monitor.
If chromatic dispersion changes due to a change in some condition, the conventional chromatic dispersion monitor searches for the optimal chromatic dispersion point using an algorithm, such as a dithering method or a down-hill method, since there is no means for determining whether the amount of chromatic dispersion compensation is excessive or insufficient. For the dithering method, see Patent document 4, and for the down-hill method, see Non-patent document 10.
For example, if an optimal point is searched from the starting point (O: point 2) shown in FIG. 5 by the down-hill method, there is no method for determining which to select, points 1 or 3 as a subsequent amount of compensation. Thus, point 1 is selected, and as a result, there is a possibility that there may be a heavy penalty. Therefore, in order to reduce the error rate, it is determined that the error rate of this point 1 is too high and point 3 is newly selected. Thus, the optimal point is gradually searched.
Next, the larger the variable amount of chromatic dispersion compensation of the variable chromatic dispersion compensator is, the shorter time is needed to optimize the amount of chromatic dispersion compensation (in FIG. 6, change units). However, a change unit is increased, the penalty caused when the amount of chromatic dispersion compensation is wrongly determined to be excessive or insufficient, also increases. Therefore, in order to compensate for chromatic dispersion with high accuracy, conventionally the change unit is set to a small value. Therefore, the prior art has an advantage of following the change with low speed (that is, a small width of change) with high accuracy, but it does not suite to follow a change with high speed (that is, a large width of change) at high speed.
The situation where the high-speed control of a wave dispersion compensator is required and its speed required then are considered below. Here, two changes; (1) a change in the chromatic dispersion value of a transmission line accompanying the switch of a route or a wavelength, and (2) a change in a transmission line chromatic dispersion value due to the change of PCD (polarization dependent chromatic dispersion) accompanying the change of higher-order PMD (polarization mode dispersion), are considered. In the case (1) above, generally, a protection time of 50 msec is a required reference control speed. Specifically, the process proceeds as follows:
Occurrence of a failure→detection of the failure→notification of the failure (=Tp≡20 msec)→switch of a route (Ts≡several msec)→main signal transmission delay (=Td)→completion of restoration
and it must be as follows:Tp+Ts+Td<50 msecThus, a time allocated to the main signal transmission delay (Td) is 20-30 msec, and this must include the control time of a variable chromatic dispersion compensator as well as the control time of EDFA. Therefore, an allowable control time including the control time of EDFA and the control time of a variable chromatic dispersion compensator is at maximum approximately 20 msec. As to case (2) above, since there is no system restriction, the control time must be at maximum 20 msec.
Next, the tunable dispersion range of the amount of chromatic dispersion compensation required by the variable chromatic dispersion compensator is considered. The automatic chromatic dispersion compensating system also compensates for the transmission line chromatic dispersion caused as time elapses. However, since the main factor of the timewise change is a change in ambient temperature in an installation environment, and its change speed is slow, the compensation for the transmission line chromatic dispersion caused as time elapses is omitted here.
Change in the chromatic dispersion of a transmission line accompanying the switch of a route or a wavelength.
Chromatic dispersion is compensated for each span, using a DCF (dispersion compensating fiber), and it is the main object of the variable chromatic dispersion compensator to compensate for the influence of its dispersion slope at the receiving end. If it is assumed that the system conditions are as follows:
One span: 100 km
Dispersion slope: 0.08 ps/nm2/km
Wavelength width to be used: 35 nm
and the remaining influence of dispersion slope is 20%, the change width Dv of a chromatic dispersion value handled by the variable chromatic dispersion compensator is at maximum as follows:
                    Dv        =                ⁢                  0.2          ×          0.08          ×          35          ×          100                                        =                ⁢                  56          ⁢                                          ⁢                      ps            /            nm                              
Change in a transmission line chromatic dispersion due to a change in PCD accompanying in a change in higher-order PMD.
Since PCD, being one element of higher-order PMD, varies depending on input polarization, a change in PCD is a phenomenon in which the chromatic dispersion of a transmission line varies. It is considered that PCD changes at high speed by fiber touch (stress is applied by touching an optical fiber by hand or blowing it). The speed of this change is generally in order of kHz, and compensation must be made at a speed of several msec.
FIG. 6 shows the experimental result of a change in the optimal amount of dispersion in a system whose overall average PMD is 8 ps (that is, 0.33 ps/4 km) and which has six spans of 100 km (100 km×6) (#1 through #7 indicate different polarized states). As shown in FIG. 6, it is observed that the optical amount of chromatic dispersion compensation changes by maximum 50 ps/nm. It is reported that the typical PMD value of an existing SMF (single mode fiber) is 0.2 ps/√km, and that the grater the influence of the first order PMD is, the greater the influence of higher-order PMD is (see Non-document 11). Therefore, it is considered that the influence of PCD in a general system is at maximum approximately 50 ps/nm.
Patent document 4: Japanese Patent Laid-open No. 2002-33701
Non-patent document 10: M. Nagaoka, “Knowledge and Prediction”, Iwanami seminar, Software science 14, Iwanami Bookstore (1988), pp.114-120
Non-patent document 11: G. Shtengel, E. Ibragimov, M. Rivera, S. suh, “Statistical Dependance between First and Second-order PMD”, No. 3, OCF2001
As a result, the following is specified:
Request for control speed: Several msec or less
Request for control range: Up to 60 ps/nm
Furthermore, if it is assumed that the allowable penalty of a chromatic dispersion compensator is 0.5 dB, and a system has the chromatic dispersion tolerance characteristic shown in FIG. 6, the adjustment accuracy is specified as follows:±10 ps/nmIts control method must have a chromatic dispersion monitor that can determine whether chromatic dispersion increases or decreases and further roughly can compute its amount instead of optimizing by repeatedly determining whether chromatic dispersion increases or decreases as in the down-hill method, and also has a function to control the amount of chromatic dispersion compensation as soon as a change occurs in chromatic dispersion.